The present invention relates to a photosensitive mixture, a photolithographic copying material produced therefrom and a process for the preparation thereof. The photosensitive mixture contains an o-quinone diazide and a binder.
It is known that o-quinone diazides are used as photosensitive substances in the production of photocopies, photoresist materials and also in the photosensitive coatings of lithographic printing plates in addition to alkaline-soluble binders which are blended with the o-quinone diazides and/or are bound to the latter chemically.
The binders generally contain phenolic hydroxyl groups and are composed preferably of oligomeric compounds with molecular weights of less than 10,000 daltons. Preferred are novolaks, in particular cresol/aldehyde condensation products or pyrogallol/acetone resins corresponding to U.S. Pat. No. 3,635,709. In addition, polyhydroxy styrene (DE-A-No. 2,322,230) and polymonomethacrylates of polyhydric phenols (DE-A-Nos. 3,528,929 and 3,528,930) are proposed as binders.
The higher the o-quinone diazide content in the mixture, the more insoluble the coating is in alkali. When the mixture is exposed to actinic radiation, the o-quinone diazide is converted into a carboxylic acid, as a result of which the alkali solubility of the coating increases. The difference in alkali solubility between exposed and unexposed regions of a photolithographic coating exposed to an image is required in order for it to be possible to develop the latter.
The content of o-quinone diazide in the mixture, however, also determines the photosensitivity of such a photolithographic mixture; and specifically, the lower the content, the higher is the expected photosensitivity. On the other hand, if the content of o-quinone diazide is too low, the difference in solubility between image areas and non-image areas is poor so that fine structures are increasingly lost in development owing to lack of differentiation.
In the two German Patents No. 875,437 and No. 893,748, positive-working photosensitive, water-insoluble diazo compounds which are applied to metallic bases to produce print forms are mixed with azo components. The azo dyestuffs produced after exposure to light are intended to make the adhesive base possible for the greasy printing ink on the printing form. In order to avoid the formation of azo dyestuffs on the areas on which light impinges, slow coupling diazo compounds are preferred. The photosensitive films do not contain a binder, and an increase in the light sensitivity as a result of the presence of coupling reagents is not observed.
In EP-A-No. 0,010,749, to improve the photosensitivity of a positive-working photosensitive mixture it is proposed to add a condensation product of a hydroxybenzophenone with formaldehyde to a mixture of an o-quinone diazide sulfonic acid ester or o-quinone diazide carboxylic acid ester or a carboxylic acid amide and an alkali-soluble and water-insoluble phenolic resin. The disadvantage is, however, an excessively soft gradation.